1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium, and a method of controlling the same.
2. Description of the Related Art
An image forming apparatus such as a printer or a copying machine using an electrophotographic method or an inkjet method is recently required to output a high-quality image. Particularly important factors that determine the quality of an output image are the tone of density and its stability. However, the density or chromaticity of an output image of an image forming apparatus varies due to the variable factors of units in the apparatus concerning environmental changes or long-time use. Note that “chromaticity” in this specification is a general term for information quantitatively representing a color. Chromaticity may be expressed as “color information” or “color value”, or simply as “color”. As a parameter to quantitatively represent a color, a general calorimetric system such as L*a*b* or XYZ can be adopted. Especially in an image forming apparatus using electrophotographic method, only a very small environmental variation may change the density or chromaticity and disturb the color balance. Hence, an arrangement for always maintaining a predetermined density is necessary.
In a current image forming apparatus, a density detection toner image (to be referred to as a patch hereinafter) of each color toner is formed on an image carrier such as an intermediate transfer member or a photosensitive member. A density sensor detects the density of each unfixed toner patch. Density control is done based on the detection result. However, the density control using the density sensor is performed by forming patches on an intermediate transfer member or a photosensitive drum and detecting them. No control is done for changes in the color balance of an image transferred and fixed on a recording medium later. That is, the density control using the density sensor cannot cope with these changes.
Japanese Patent Application Laid-Open No. 2003-107833 proposes an image forming apparatus which includes a sensor (to be referred to as a color sensor hereinafter) to detect the density or chromaticity of a patch formed on a recording medium and provides an image having excellent color reproductivity by correcting the density or chromaticity of a toner image based on a measurement result. The color sensor uses, as light-emitting elements, three or more kinds of light sources having different emission spectra such as red (R), green (G), and blue (B). Alternatively, the color sensor uses a light source for emitting white (W) light as a light-emitting element and includes three or more kinds of filters such as red (R), green (G), and blue (B) filters which have different spectral transmittances and are formed on the light-emitting element. The color sensor having such an arrangement can obtain three or more different outputs such as R, G, and B outputs.
FIG. 13 is a view showing an example of a patch array 1300 formed on a recording medium to correct color balance. A color sensor is designed to detect the patch array 1300 before the recording medium is discharged out of the apparatus. Generally, the color sensor starts detection when the recording medium has reached the color sensor. After detecting the first patch, the color sensor sequentially detects the patches at a predetermined timing, thereby obtaining the detection data of each patch.
In the above-described related art, however, when detecting the patches at a predetermined timing, the color sensor may detect a patch having a tone different from an assumed tone because of operation variations of the constituent elements caused by changes over time or environmental changes. If this situation occurs, the color balance correction accuracy degrades. The operation variations include, for example, variations in the outer diameter of a recording medium conveyance roller, and variations in the recording medium conveyance speed caused by, for example, environmental variations. The operation variations also include shrinkage of the recording medium that has passed through a fixing device, and expansion and contraction of an image until image formation on the recording medium.
To avoid the influence of these operation variations, it is necessary to determine the length of each patch to be used for color balance correction. More specifically, a sufficiently long patch needs to be set to enable reliable patch detection even in the presence of variations. For example, to cause an image forming apparatus using a color sensor to output a high-quality image, the number of patches must be increased to improve the color balance correction accuracy.
However, when the number of patches to be used for color balance correction, the conveyance-direction length of the recording medium, or the conveyance speed of the recording medium increases, toner image portions including margins must be provided at the leading and trailing edge portions of each patch. This leads to a waste of printing media and toners.
A predetermined time is necessary for the color sensor to detect one patch. For this reason, the patch conveyance-direction length must have a predetermined value or more. More specifically, when the number of patches to be used for color balance correction is increased, not all patches are already formed on one recording medium. Additionally, as the throughput of the image forming apparatus improves, the conveyance-direction length of one patch must be longer. Hence, the number of patches per recording medium decreases, and printing media and toners are consumed in large quantities at the time of color balance correction.
The recording medium having the patches for color balance correction is unnecessary for the user. Hence, printing media and toners are preferably used in smaller quantities.